I am the proud owner of a '72 220D that was purchased new by my grandparents. The car runs great and is a pleasure, but lately with the newly cold weather, it's been quite a challenge to start. My father seems to think it's related to the fuel injection system or the fuel pump. Does that sound about right? Also, starting today, the key won't turn in the ignition to power the car so I can start it. (Everything's right; it's in park, etc.) Any answers or suggestions would be appreciated. Also, there is a '67 250SL (silver, navy blue hardtop) for sale in my family if anyone has any interest.

if it fells like the key wont turn it could be the key its self. if it feels like the key is pushing the pins out of the tumbler(you can feel it move a little). in my experience with locks you should try to avoid putting liquid lubricants into locks as they get sticky and hang up the pins in the long run. try to remove the whole tumbler and then work with it using permatex dry lube with teflon. also, i am sure you tried this, but if you turn the wheel with the key out of the ignition, the steering wheel locks up. move the wheel about a little while turning the key gently..
good luck

if your key is original - it could be worn down to the point where it won't activate the tumblers. I had that problem on an old North American car. I found an innovative locksmith who cut a new key using a piece of match book cover under the key to raise up the key slightly in the jig before cutting. In my case, it cost me two keys as he had to use 2 pieces of match cover to get it up high enough to engage the tumblers. Before tearing out the ignition assy. it might be worth a try.